1. Technical Field
This invention concerns the field of filtration and more specifically, rotary disc filtration devices.
2. Background Art
Filtration devices are used to separate one or more components of a fluid from other components. Common processes carried out in such devices include classic filtration, microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, pervaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, and bacteriological filtration. As used herein, the term "filtration" includes all of those separation processes as well as any other processes using a filter that separate one or more components of a fluid from the other components of the fluid.
Filtration processes make use of the greater filter permeability of some fluid components than others. As used herein, the term "filter" includes any article made of any material that allows one or more components of a fluid to pass through it to separate those components from other components of the fluid. Thus, the term "filter" includes metallic and polymeric cloth filters, semipermeable membranes and inorganic sieve materials (e.g., zeolites, ceramics). A filter may have any shape or form, for example, woven or non-woven fabrics, fibers, membranes, sieves, sheets, films, and combinations thereof.
The components of the fluid that pass through the filter comprise the "permeate"and those that do not pass (i.e., are rejected by the filter or are held by the filter) comprise the "retentate." The valuable fraction from the filtration process may be the retentate or the permeate or in some cases both may be valuable.
A common technical problem in all filtration devices is blinding or clogging of the filter. Permeate passing through the filter from the fluid layer adjacent to the feed side of the filter leaves a retentate layer adjacent to or on that side of the filter having a different composition than that of the bulk feed fluid. This material may bind to the filter and clog its pores (that is, foul the filter) or remain as a stagnant boundary layer, either of which hinders transport of the components trying to pass through the filter to the permeate product side of the filter. In other words, mass transport per unit area through the filter per unit time (i.e., flux) is reduced and the inherent sieving capability of the filter is adversely affected.
Generally, fouling of the filter is chemical in nature, involving chemisorption of substances in the feed fluid onto the filter's internal (pore) and external surface area. Unless the chemical properties of the filter surface are altered to prevent or reduce adsorption, frequent and costly filter replacement or cleaning operations are necessary.
One of the most common causes of fouling arises from the low surface energy (e.g., hydrophobic nature) of many filters. U.S. Pat. Nos. 4,906,379 and 5,000,848, which are assigned to Membrex, Inc., assignee of the present application, disclose chemical modification to increase the surface free energy (e.g., hydrophilicity) of filter surfaces. (All of the documents identified, discussed, or otherwise referenced in this application are incorporated herein in their entirety for all purposes.) In general, however, relatively little attention has been given to modifying surface chemistry to reduce filter fouling.
In contrast to the chemical nature of most fouling problems, the formation of a boundary layer near the surface of the filter is physical in nature, arising from an imbalance in the mass transfer of feed fluid components towards the filter surface as compared to the back-transfer from the boundary layer to the bulk feed fluid. Some form of force (for example, mechanical, electro-kinetic) must be used to promote the desired mass transfer away from the filter surface. Unfortunately, few strategies have been developed that promote adequate back-mixing to reduce the boundary layer or prevent its formation.
The most common strategy is called "cross-flow" filtration ("CFF") or "tangential flow" filtration ("TFF"). In principle, the feed fluid is pumped across (i.e., parallel to) the outer surface of the filter at a velocity high enough to disrupt and back-mix the boundary layer. In practice, however, cross-flow has several disadvantages. For example, equipment must be designed to handle the higher flow rates that are required, and such higher flow rates generally require recirculating retentate. However, recirculation can injure certain materials that may be present in the fluid (e.g., cells, proteins) and make them unsuitable for further use (e.g., testing).
A different approach to eliminating the stagnant boundary layer involves decoupling the feed flow rate from the applied pressure. With this approach, a structural element of the filtration device, rather than the feed fluid, is moved to effect back-mixing and reduction of the boundary layer. The moving body may be the filter itself or a body located near the filter element.
Some of the rare moving-body devices that have enhanced filtration without energy inefficient turbulence are exemplified in U.S. Pat. No. 4,790,942, U.S. Pat. No. 4,876,013, and U.S. Pat. No. 4,911,847 (assigned to Membrex, Inc.). These three patents each disclose the use of filtration apparatus comprising outer and inner cylindrical bodies defining an annular gap for receiving a feed fluid. The surface of at least one of the bodies defining the gap is the surface of a filter, and one or both of the bodies may be rotated. Induced rotational flow between these cylinders is an example of unstable fluid stratification caused by centrifugal forces. The onset of this instability can be expressed with the aid of a characteristic number known as the Taylor number. Above a certain value of the Taylor number, a vortical flow profile comprising so-called Taylor vortices appears. This type of secondary flow causes highly efficient non-turbulent shear at the filter surface(s) that reduces the stagnant boundary layer thickness and, thus, increases the permeate flux.
In contrast to classic cross-flow filtration, the devices of U.S. Pat. No. 4,790,942, U.S. Pat. No. 4,876,013, and U.S. Pat. No. 4,911,847 allow the shear rate near the filtration surface and the transmembrane pressure to be independently controlled. Furthermore, because those two operating parameters are independent and high feed rates are not required to improve the permeate flux, the feed rate can be adjusted to avoid non-uniform transmembrane pressure distributions. Accordingly, mechanically agitated systems of this type enable precise control over the separation.
Rotary disc filtration devices also allow shear rate near the filtration surface and transmembrane pressure to be independently controlled. In such devices feed fluid is placed between the disc and oppositely disposed filtration surface that define the fluid filtration gap and one or both of the disc and filtration surface are rotated. See, e.g., U.S. Pat. No. 5,143,630 and 5,254,250 (both assigned to Membrex, Inc.). Additional documents concerning rotating impellers, rotary discs, filtration, rotary disc filtration devices, other filtration devices using mechanical agitation, and seals include: U.S. Pat. No. 1,762,560; U.S. Pat. No. 3,455,821; U.S. Pat. No. 3,477,575; U.S. Pat. No. 3,884,813;
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Conventional rotating disc filter devices utilize stacked filter disc arrangements. Historically, most of these devices comprise disc filters that are rotated by a central drive shaft to which the filter elements are attached. Some rotating disc devices utilize stationary filter discs separated from each other by rotary elements attached to the shaft. Murkes and Carlsson, Crossflow Filtration--Theory and Practice, John Wiley & Sons, New York (1988), FIG. 3.15 at page 91. In this type of device a unitary stationary filter element surrounds the central rotating drive shaft.
The effectiveness of rotating disc filtration devices depends in large part upon the flowpaths of the feed, retentate, and permeate fluids. Means to overcome the potential for buildup of rejected species caused by flowpath limitations may involve changing either the rotating disc design (e.g., adding blades or grooves), or changing the feed pathways, or both. In some designs, feed fluid is introduced near the peripheries of the filter(s) and disc(s). In other designs, feed fluid is introduced near the axis of rotation (longitudinal axis of the filter(s) and disc(s)) and the feed fluid may be delivered to the fluid filtration gap(s) via hollow rotating shafts having ports (or nozzles) to direct the feed to either or both sides of the filter support members.
It has been found that in some cases during use of a rotary disc filtration device, the disc and its adjacent filter defining the fluid filtration gap may contact one another, which is highly undesirable (e.g., the "binding" or "rubbing" of disc against filter may significantly increase power requirements, the filter may be harmed, and the rotary bearings may suffer premature wear or failure). Despite all the development work concerning rotary disc filtration devices, the need still exists for rotary disc filtration devices that can avoid such contact and the ensuing problems.